1. Field of the Invention
This invention relates to a projection optical system, and in particular to an apparatus for correcting an optical characteristic thereof.
2. Description of the Prior Art
Reduction projection type exposure apparatuses called steppers have been much introduced into the production field of super-LSI in recent years and have been crowned with great success, and resolving power may be mentioned as one of the important performances thereof. One of important factors which affect this resolving power is the positional deviation of the imaging plane of the projection optical system in the direction of the optic axis. The size of the pattern used in super LSI has been strengthening its tendency toward minuteness year by year and correspondingly, the need for the improvement of the resolving power has been strengthening. Accordingly, the necessity of keeping the imaging plane at a predetermined position has become very high.
The position of the imaging plane of the projection optical system is adjusted during the manufacture or assembly of the apparatus to such a degree that the imaging plane can be neglected. However, the projection lens of the stepper absorbs part of the exposure energy and rises in temperature. Therefore, if light energy continues to be applied to the projection lens for a long time or the exposure operation is continuously performed for a long time, the position of the imaging plane moves to a significant degree. Likewise, the magnification of the projection lens also varies. Therefore, in a stepper used for the exposure of more minute patterns, those fluctuations must be made sufficiently small.